Add installSdk() and promptSdk() to the installer so users can
optionally install @gsd-build/sdk during GSD setup. The --sdk flag
installs without prompting; interactive installs get a Y/N prompt
after runtime installation completes. SDK installs use @latest with
suppressed npm noise (--force --no-fund --loglevel=error, stdio: pipe).
Cherry-picked from fix/sdk-cli-runtime-bugs (de9f18f) which was
left out of #1407.
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
a858c6ddff